17 February 2011

Songs and size in sabrewings: evolution across an isthmus

The name sounds like a super top secret stealth fighter jet. The kind that make deep swooshing noises as it flies by that you can barely hear but feel in your bones.

But it’s even cooler than that. The wedge-tailed sabrewing is a tropical American hummingbird. And we know how cool hummingbirds are.

This particular hummingbird species has a couple of extra cool features, though, mostly revolving around mating. The males form leks, loose groups of males that show off for females. They also have elaborate songs, which is quite variable.

We know hummingbirds are quick, but are they quick evolving birds, too? Clementina González and colleagues wanted to find out. They were particularly interested because this bird’s distribution straddles the Isthmus of Tehuantepec in southern Mexico, which is the narrowest point between the Gulf of Mexico and the Pacific Ocean. The isthmus divides two mountain ranges.

The hummingbird populations are not restricted to the high mountain ranges, but they are somewhat separated in the isthmus. For evolutionary biologists, variation of the species and disconnected populations virtually screams, “Allopatric speciation!” Heck, the conditions for these hummingbirds are virtually the definition of allopatric speciation.

When they looked at the DNA of the three subspecies, bird, they were able to cluster them neatly into one eastern and two western populations, on either side of the isthmus. The three different subspecies could also be distinguished by their the songs.

When looking at the bodies of the sabrewings, one of the western subspecies stood out from either of the other two by being larger. The other two were a little more difficult to distinguish based on their body shape, although bill size could pick apart the remaining two subspecies.

With all these different groups of data, they then tried to figure out what caused the differences in these populations. For the differences in morphology, genetic drift alone – random mutations accumulating in one population because they don’t mate with the others – seemed to explain the differences.

The songs, however... they seemed to be not randomly wafting apart as time went on. The changes were such that the authors suggested that selection was occurring to shape the songs of the different populations.

Songs can be shaped over evolutionary time by many different selective pressures. Physics is one. If you move into different habitats with different acoustical properties, different sounds are advantageous. González and colleagues didn’t find any evidence for that, though.

González and colleagues suggest that song evolution is being driven by female preferences for particular songs. This seems plausible on the face of it, but the authors also note that hummingbirds are like humans, songbirds, and parrots: they can learn their vocalizations.

Differences in songs might be better described as changes in learned behaviour (cultural drift, if you will) than evolutionary selection. It’s not clear to me at this instant how you might distinguish between the two alternatives experimentally.

The largest of these three subspecies is also the one in the most danger. It tends to live more in the lowlands, which is being logged for timber.